Date:	Tue, 19 Mar 1996 13:50:25 +0800 (GMT+0800)
From:	"Orlando A. Andico" <orly AT natalie DOT eee DOT upd DOT edu DOT ph>
To:	Christopher A Triebel <ctriebel AT kepler DOT unh DOT edu>
cc:	DJGPP Mailing List <djgpp AT delorie DOT com>
Subject:	Re: Graphics modes
In-Reply-To:	<Pine.ULT.3.91a.960318125248.4068C-100000@kepler.unh.edu>
Message-ID:	<Pine.LNX.3.91.960319134302.203A-100000@natalie.eee.upd.edu.ph>
MIME-Version:	1.0

On Mon, 18 Mar 1996, Christopher A Triebel wrote:

<snip>
> a specialist on DJGPP but if I understand what the non-interlaced means 
> it is the way that the video card stores and retrieves information for 
> the user.  In non-interlaced mode all memory locations are stored 
> contiguously straight through.  In interlaced ever other screen line is 
> contigous.  In other words if you have a video buffer 64K in size the 
> first 32K contains every odd numbered line, and the second 32K is every 
> even numbered line.  Or vice versa.  I don't have a text handy, but I 
> think that CGA used that approach.
>
<snip>

True, CGA *did* use an interlaced memory scheme. But DJGPP doesn't, and 
never, supported CGA. The "largest interlaced mode" has to do with the 
HSYNC of your monitor and your card. For example, the original IBM 8514/A 
video card supported 640x480 and 1024x768 (in 1987 that was pretty good). 
The catch was, most monitors didn't have enough bandwidth for 1024x768, 
so IBM solved this problem by interlacing -- on pass 1, the even-numbered 
rows are drawn, on pass 2, odd-numbered rows. This is on the *monitor*, 
it has nothing to do with memory organization.

The NTSC (ordinary TV) specification also uses interlacing to fit all 
that signal bandwidth into the 4-MHz TV channel (if you think a stock VGA 
monitor with it's 18-MHz bandwidth is bad, just imagine how bad our MTV 
is!!!)

The disadvantage of interlacing is that it flickers badly because the 
screen refresh rate is effectively halved. Look really close at your TV 
screen and you'll notice that the pixels are flashing slowly enough for 
you to see. For monitors, the flashing ain't that bad, but if your 
graphics screen is mostly light-colored, you'll notice it. This is why 
the "largest non-interlaced mode" is important; most people wouldn't care 
much for ultra-high resolution but with eye-tearing flicker.

These days though, with the VESA standard refresh rate of 72 Hz, 
interlacing is getting pretty rare (the monitors are fast enough) but you 
also need a fast card!!! I've only seen interlacing at 1280x1024 
resolutions (on a multiscanning SVGA monitor).

Cheers,

                                             ------------------------
                                              Orlando A. Andico
                                              oandico AT eee DOT upd DOT edu DOT ph
                                             ------------------------

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